This invention relates to tooling which is employed in the casting of solid rocket propellants.
A solid propellant rocket generally employs a rocket motor case, insulation, a liner composition, and a solid propellant grain in the order listed as viewed from the outer motor case to the solid propellant grain contained therein. The functions of each of the components of a solid propellant rocket motor are well defined in the art.
The solid propellant grain may be processed by extrusion or casting techniques. The majority of modern composite propellants for rocket motors are cast directly into the case as a cross-linkable mixture and cured in place.
The thrust-time characteristic of a solid propellant rocket can be controlled by the geometric shape of the grain. Neutral burning grains maintain a constant surface during burning and produce a constant thrust. Progressive burning grains increase in surface during burning and produce an increasing thrust with time. Regressive burning grains decrease in surface during burning and produce a decreasing thrust with time.
The geometric shape of a grain is generally understood to be the shape of the internal perforation or perforations. Internally perforated, outwardly burning grains are superior to end burning or external burning grains because the wall area of the motor case is protected from the hot gas generated by combustion.
The perforation in a cast solid propellant grain is produced by casting the cross-linkable mixture around shaped casting tooling, curing the mixture and withdrawing the tooling from the cured solid propellant. Glass-filled polytetrafluoroethylene casting tooling is used extensively throughout the solid propellant industry to mold or form the solid propellant grain of rocket motors. From both the safety and design standpoints, it is an ideal material of construction. It is a relatively soft material with a low compressive strength, yet is dimensionally stable. Unfortunately, the mold release properties of this material are reduced as glass filler is added. The mold release properties also degrade with repeated use, because mobile propellant species migrate into the porous glass-filled polymer substrate. Conventional mold release agents are ineffective with this tooling since they are not impervious and allow species migration into the glass-filled polymer substrate. Conventional cleaning methods are also ineffective since they do not remove subsurface contamination. Thus, repeated usage of glass-filled polytetrafluoroethylene casting tooling results in removal difficulties and damage to the propellant grain in the form of surface tears.
Accordingly, it is an object of this invention to provide a method for modifying solid propellant casting tooling to provide improved mold release.
It is another object of this invention to provide improved solid propellant casting tooling.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.